Field of the Invention
The present disclosure relates to the field of power electronics, and more particularly, to a supply voltage generating circuit and a switching power supply.
Description of the Related Art
With rapid development in electronic information industry, switching power supplies are widely utilized in various fields, such as computers, electrical equipment, instrumentation, light-emitting diodes (LEDs), medical devices, military equipment and so on. A conventional switching power supply has a circuit structure including a power stage circuit and a control circuit and a supply voltage generating circuit which supplies electric energy to the control circuit.
FIG. 1A and FIG. 1B are schematic diagrams showing a conventional switching power supply, where a dashed box indicates a supply voltage generating circuit.
As shown in FIG. 1A, the switching power supply includes a transformer consisting of a primary winding L1 and a secondary winding L2, a first diode D1, a load-side capacitor COUT, a first switch Q1, a first resistor RS, a control circuit 101, a power capacitor CVCC, an auxiliary winding L3, and a second diode D2. Here, the switching power supply is used for supplying electric energy to an LED device 102. The main circuit has a flyback isolation-type topology, in which the supply voltage generating circuit for supplying electric energy to the control circuit 101 includes the auxiliary winding L3 electrically coupling to the transformer, the second diode D2 and the power capacitor CVCC. The power capacitor CVCC has one terminal being electrically coupled to a non-dotted terminal of the auxiliary winding and the other terminal being electrically coupled to the dotted terminal of the auxiliary winding through the second diode D2. The supply voltage generating circuit receives electric energy from the power stage circuit by means of the auxiliary winding, charges the power capacitor CVCC, and then provides a supply voltage VCC to the control circuit 101 by means of the power capacitor CVCC.
However, the transformer has a complex structure and increased manufacture cost because the auxiliary winding L3 must be included in the circuit.
As shown in FIG. 1B, the switching power supply includes a source-side driving circuit which is self-powered. The switching power supply includes a first inductor L, a first diode D1, an output capacitor COUT, a second switch Q2, a first switch Q1, a second diode D2, a control circuit 101, and a power capacitor CVCC. The supply voltage generating circuit in the switching power supply includes the second switch Q2 and the second diode D2. When the supply voltage generating circuit supplies electric energy to the control circuit 101, voltage variation at a source side of the second switch Q2 is utilized for controlling on and off states, without the need for an auxiliary winding. When the second switch Q2 is turned on, the power capacitor CVCC is charged through a drain-source parasitic capacitance of the second switch Q2 and through the second diode D2, and then supplies electric energy to the control circuit 101.
However, the circuit scheme shown in FIG. 1B has the disadvantage that it may not be able to provide the supply voltage for the control circuit 101, because the second switch Q2 has a drain-source parasitic capacitance too small to ensure that the power capacitor CVCC is charged to a large voltage value when an input voltage VIN has a small value. Moreover, the second switch Q2 has a source being grounded through the first switch Q1, and as a result, it cannot be turned off completely. The circuit also has the problem of easily resonating in actual operation.